Metallic structure with coupled network to reduce back scattering

ABSTRACT

The invention suppresses undesired radiation from objects having more than one dimension that is a significant fraction of a wavelength of energy exchanged with nearby radio antennas. Such a structure might be a hollow cylinder of height and transverse dimensions (diameter, if circular) that are a substantial fraction of a wavelength supported above and insulatedly separated from a ground plane by four terminals in space quadrature. At least two independently adjustable reactances are connected to the cylinder at points which are separated at some distance in the direction of wave propagation. If waves are expected to arrive from any direction, then a minimum of three independent reactances are required, although four can be used, for example, one connected to each of the terminals in space quadrature.

United States Patent Inventor Appl. No. Filed Patented METALLIC STRUCTURE WITH COUPLED NETWORK TO REDUCE BACK SCATTERING 6 Claims, 2 Drawing Figs.

US. Cl 343/885, 343/750 Int. Cl l-l0lq l/50 Field of Search 343/885,

References Cited UNITED STATES PATENTS FRONT /7 TERMINAL DIRECTION OF PROPAGATION REACTIVE LINE STRCHER Primary Examiner-Eli Lieberman Attorney-Wolf, Greenfield and Sacks ABSTRACT: The invention suppresses undesired radiation from objects having more than one dimension that is a significant fraction ofa wavelength of energy exchanged with nearby radio antennas. Such a structure might be a hollow cylinder of height and transverse dimensions (diameter, if circular) that are a substantial fraction of a wavelength supported above and insulatedly separated from a ground plane by four terminals in space quadrature. At least two independently adjustable reactances are connected to the cylinder at points which are separated at some distance in the direction of wave propagation. If waves are expected to arrive from any direction, then a minimum of three independent reactances are required, although four can be used, for example, one connected to each of the terminals in space quadrature.

SIDE TERMINAL PATENTEDHUVZ 1971 3,618; 1 13 SIDE TERM! NAL FRONT TERMINAL GROUND PLANE E FRONT SIDE TERMINAL /7 TERMINAL r /5 BACK TERMINAL DIRECTION OF PROPAGATIO V E L.

L51 GROUND PLANE 1 H Y FROM sIOE 8 TERMINAL 2.2 REAOT IvE REAcTIvE I 26 LINE LINE If Q sTRETcI-IER sTRETcIIER a A REAOTII/E h REAOTNE LINE 65 LINE S'TREEQHER 56 STREECHER IN VENTUR, AN DREW ALF-OR D ATTOR N EYS METALLIC STRUCTURE WllTll-I COUPLED NETWORK TO REDUCE RACK SCATTERING This is a continuation of application Ser. No. 516,373 filed Dec. 27, 1965, now abandoned.

The present invention relates in general to detuning and more particularly concerns apparatus and techniques for detuningmultiple electromagnetic radiating modes in structures that may cause substantial undesired scattering.

It is known that tall structures in the vicinity of radio antennas have caused substantial scattering to effectively alter the desired radiation pattern of the neighboring radio antennas. Application of detuning techniques, such as those disclosed in Alford U.S. Pat. No. 2,61 1,871 have significantly reduced scattering in the case of other radio and transmission line towers by coupling reactance networks to the disturbing towers at a single point ofeach tower in such a manner so as to eliminate significant undesired reradiation. Detuning techniques have also been applied to thin cylindrical objects. But each of these known techniques has been concerned only with suppressing radiation of an undesired fundamental mode wherein only one dimension of the object is a significant fraction ofa wavelength.

Accordingly, it is an important object of this invention to suppress undesired radiation from. objects having more than one dimension that is a significant fraction ofa wavelength.

It is an important object of this invention to achieve such suppression with the addition of relatively few additional elements.

It is a further object of the invention to achieve suppression of radiation ofmore than one mode from such an object.

It is still a further object of the invention to provide methods and means for effecting detuning of multiple modes whereby detuning of one mode may be effected independently of the detuning ofthe other mode.

According to the invention, an object has at least two dimensions that are at least a significant fraction of a quarter wavelength. Means are provided for coupling a plurality of reactive elements to spaced points of the object to suppress radiation from the object of more than one mode. In a preferred form of the invention, the means for coupling comprises hybrid means having a pair of side branches and sum and difference (parallel and series) branches. Respective ones of the side branches are coupled to spaced points on the object and the sum and difference branches are coupled to respective detuning reactances.

Numerous other features, objects and advantages of the invention will become apparent from the following specification when read in connection with the accompanying drawing in which:

FIG. 1 is a perspective view of a hollow cylinder above a ground plane support from four terminals in space quadrature helpful in understanding the principles of the invention; and

FIG. 2 is a view through an axial section of the structure of FIG. 1 illustrating how hybrids may be employed to couple reactive elements to the structure and suppress both fundamental and second radiation modes.

With reference now to the drawing and more particularly FIGS. l and 2 thereof, there is shown a cylinder llll supported above and insulatedly separated from a ground plane 12. by four terminals 113, M, and 16 in space quadrature. The diameter (and, hence, the width and length) of the hollow cylinder H and its height are a substantial fraction of a wavelength.

An electromagnetic field having a component 17 perpendicular to the ground plane and parallel to the axis of cylinder 11 induces at least two modes of currents in the cylinder. In the first or fundamental of these, mode A, currents of uniform density flow in the conducting sheet of the cylinder 11 parallel to its axis. In the second order mode, mode B, the currents have components parallel to the axis of the cylinder but of nonuniform density and of opposed phases on the front and back walls.

The current density of the B mode component parallel to the axis ofthe cylinder is at its maximum at the front and back walls and zero or near zero on the sidewall when either the front or back walls are facing the source ofthe inducing field.

The known method for detuning the cylinder 11 would require that the cylinder 11 be terminated in a bottom-conducting plate whose center is connected through an adjustable reactance to the ground plane 12. And this known method works well when the diameter of the cylinder (and, hence, its length and width) is small in comparison with a quarter wavelength because substantially only fundamental mode A currents are induced.

However, when the diameter of the cylinder becomes comparable with a quarter wavelength, secondary mode B currents become appreciable in comparison with fundamental mode A currents. Since the current density of mode B currents is essentially zero at the center of the cylinder it has been discovered that these currents continue to flow essentially unaffected by a single adjustable reactance placed between the center of the bottom plate of the cylinder 11 and the ground plane 112.

It has been discovered that when currents in secondary mode B as well as in fundamental mode A are substantial, it is necessary to have at least two independently adjustable reactances connected to the cylinder at points which are separated at some distance in the direction of wave propagation. If waves are expected to arrive from any direction, then a minimum of three independent reactances are required. although four can be used. For example, one may connect reactances at the front terminal 13, side terminals 14 and I6 and back terminal 115.

The detuning process will be better understood by considering the convenient means for achieving the detuning shown in FIG. 2. There a hybrid 21 is shown with its side branches I and II respectively connected by equal lengths of transmission line to front terminal 13 and back terminal 15, these terminals being connected to points on the cylinder at opposite extremes along the direction of propagation.

Hybrid 21 has the following properties when its four branches are terminated in their characteristic impedance. Energy applied to the P or sum branch divides equally between side branches I and II with virtually no energy reaching the S or difference branch. Energy applied to the S branch also divides equally between side branches I and II with virtually no energy reaching the P branch. Energy of equal magnitude and phase coincidence applied to branches I and II is delivered to the P or sum branch with virtually none of such energy reaching the S or difference branch. Energy of equal magnitude but opposite phase applied to side branches 1 and II is delivered to the S or difference branch while the P or sum branch receives virtually none of such energy.

As a result ofthese hybrid properties the fundamental mode A current causes signals of substantially equal magnitude and phase coincidence to be applied to side branches 1 and II with virtually all of this energy appearing at the P or sum branch. Since the secondary mode B branch currents at front terminal 13 and back terminal 15 are of substantially equal magnitude but opposite phase, they produce substantially equal mag nitude opposite phase signals at side branches I and II with the result that the mode B energy is delivered to the S or difference branch. it was found that the backscatter from the cylinder 11 could be very substantially decreased by connecting one line stretcher 22 to the P or sum branch, another line stretcher 23 to the S or difference branch and adjusting the lengths of the line stretchers for a minimum of backscatter. The line stretchers may be short-circuited or open-ended at their far ends. This arrangement enables one to effectively introduce reactances in the path of the currents flowing in both modes A and B.

When the inducing field may arrive from a direction at any azimuth around the cylinder (assuming for a moment that the axis of the cylinder is vertical), a second hybrid 24 is shown with its side branches 1 and II connected to side terminals 14 and 16, respectively, by substantially equal length transmission lines and its P branch coupled to a reactive line stretcher A 25 and its S branch coupled to a reactive line stretcher B. The four line stretchers 22, 23, 25 and 26 may then be adjusted until both fundamental A mode scattering and secondary mode B scattering is minimized. It has been found that the P branches of the two hybrids 21 and 24 may be connected together by means of a tee fitting so that one common line stretcher may be used for suppressing the fundamental A mode. The difference branches, however, control two modes of currents in mutually effectively orthogonal systems which must be controlled separately. Thus, it has been found that two separate line stretchers 23 and 26 are required to suppress the secondary B mode when radiation may originate from any direction.

When the cylinder is of still larger diameter, third and other higher order modes are introduced. Then more and more independent tuning branches must be connected to the cylinder at spaced points in order to be able to detune the currents in the various modes.

it has been found that bodies of shapes other than a cylinder exhibit the scattering properties discussed above. For example, it was found that one second order mode of current can be induced even in a thin plate provided that the direction of propagation is not at right angles to the plate. The principles described herein are applicable to suppressing such modes. For example, if cylinder 11 were shrunk into a plate between front terminal 13 and back terminal 15, hybrid 21 and line stretchers 22 and 23 would be sufficient to suppress the fundamental and secondary modes regardless of the azimuth direction from which its energy approaches.

While the hybrid means for coupling the adjustable reactances is especially convenient, it should be noted that suppression could be achieved by placing independently adjustable reactances directly between the lower edge of cylinder 11 and ground plane 12. For example, three adjustable reactances spaced 120 about cylinder 11 could, when properly adjusted, suffice to substantially suppress fundamental and secondary mode currents in cylinder 11.

The foregoing exemplary embodiments and variations thereof set forth are by way of example only and not as a limitation on the invention. it is evident that those skilled in the art may now make 'numerous uses and modifications of and departures from the specific embodiments described herein without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every feature and novel combination of features present in or possessed by the apparatus and techniques herein disclosed and to be construed as limited solely by the spirit and scope of the appended claims.

What is claimed is:

1. A conducting structure having means for detuning comprising,

a conducting structure having at least two dimensions that are a significant fraction of a quarter wavelength at a predetermined frequency and characterized by a plurality of electromagnetic radiating modes,

a plurality of reactive elements for suppressing at least fundamental and secondary ones of said electromagnetic radiating modes,

and means for coupling said reactive elements to said conducting structure at spaced points thereof to suppress at least said fundamental and secondary ones of said modes, wherein said means for coupling comprises,

a first hybrid means having a parallel branch, a series branch and first and second side branches, means for coupling said first and second side branches to first and second ones respectively of said spaced points, means for coupling a first of said reactive elements to one of said series and parallel branches, means for coupling a second of said reactive elements to the other of said series and parallel branches, a second hybrid means having a parallel branch, a series branch and first and second side branches, means for coupling said second hybrid means first and second side branches to third and fourth ones respectively of said spaced points,

and means for coupling at least a third of said reactive elements to one of said second hybrid means series and parallel branches. 1

2. A conducting structure having means for detuning in accordance with claim 1 and further comprising,

means for coupling a fourth of said reactive elements to the other of said second hybrid means series and parallel branches.

3. A conducting structure in accordance with claim 1 wherein said third reactive element is coupled to said second hybrid means series branch, said second reactive element is coupled to said first hybrid means parallel branch and further comprising,

means for coupling said second reactive means to said second hybrid means parallel branch.

4. A conducting structure having means for detuning in accordance with claim 3 wherein said first and second ones of said spaced points are located at opposite ends of a first line corresponding substantially to at least one of said dimensions,

said third and fourth ones of said spaced points are located at opposite ends of a second line substantially corresponding to at least another ofsaid dimensions,

said means for coupling said first hybrid means side branches to said first and second ones of said spaced points comprise transmission line means providing substantially equal effective electrical pathlengths between each side branch and the associated one of said first and second spaced points,

and said means for coupling said second hybrid means side branches to said third and fourth ones of said spaced points comprise transmission line means providing substantially equal effective electrical pathlengths between each side branch and the associated one of said third and fourth spaced points.

5. A conducting structure having means for detuning in accordance with claim 4 wherein said reactive elements are adjustable line stretcher means.

6. A conducting structure having means for detuning in accordance with claim 4 wherein said first and second line comprise respective orthogonal diameters of said conducting structure. 

1. A conducting structure having means for detuning comprising, a conducting structure having at least two dimensions that are a significant fraction of a quarter wavelength at a predetermined frequency and characterized by a plurality of electromagnetic radiating modes, a plurality of reactive elements for suppressing at least fundamental and secondary ones of said electromagnetic radiating modes, and means for coupling said reactive elements to said conducting structure at spaced points thereof to suppress at least said fundamental and secondary ones of said modes, wherein said means for coupling comprises, a first hybrid means having a parallel branch, a series branch and first and second side branches, means for coupling said first and second side branches to first and second ones respectively of said spaced points, means for coupling a first of said reactive elements to one of said series and parallel branches, means for coupling a second of said reactive elements to the other of said series and parallel branches, a second hybrid means having a parallel branch, a series branch and first and second side branches, means for coupling said second hybrid means first and second side branches to third and fourth ones respectively of said spaced points, and means for coupling at least a third of said reactive elements to one of said second hybrid means series and parallel branches.
 2. A conducting structure having means for detuning in accordance with claim 1 and further comprising, means for coupling a fourth of said reactive elements to the other of said second hybrid means series and parallel branches.
 3. A conducting structure in accordance with claim 1 wherein said third reactive element is coupled to said second hybrid means series branch, said second reactive element is coupled to said first hybrid means parallel branch and further comprising, means for coupling said second reactive means to said second hybrid means parallel branch.
 4. A conducting structure having means for detuning in accordance with claim 3 wherein said first and second ones of said spaced points are located at opposite ends of a first line corresponding substantially to at least one of said dimensions, said third and fourth ones of said spaced points are located at opposite ends of a second line substantially corresponding to at least another of said dimensions, said means for coupling said first hybrid means side branches to said first and second ones of said spaced points comprise transmission line means providing substantially equal effective electrical pathlengths between each side branch and the associated one of said first and second spaced points, and said means for coupling said second hybrid means side branches to said third and fourth ones of said spaced points comprise transmission line means providing substantially equal effective electrical pathlengths between each side branch and the associated one of said third and fourth spaced points.
 5. A conducting structure having means for detuning in accordance with claim 4 wherein said reactive elements are adjustable line stretcher means.
 6. A conducting structure having means for detuning in accordance with claim 4 wherein said first and second line comprise respective orthogonal diameters of said conducting structure. 